Wire T212 pagination, retries, and click<8.2 pin

Context
-------
Closes out the Trading212 provider's retry + pagination surface so
the "Add Trading212Provider core fetch" commit has everything the
CronJob needs: cursor-based pagination, 429 honouring Retry-After,
jittered exponential backoff for 429-without-header and 5xx, bailout
after _MAX_RETRIES, and checkpoint-after-page semantics so a crashed
run resumes at the start of the unfinished page.

Also pins click<8.2 — typer 0.12 calls Parameter.make_metavar()
without a ctx argument, which click 8.2 removed; `broker-sync --help`
was crashing with TypeError until this pin. typer 0.15+ would also
fix it; the pin is lower friction.

One test fix: test_checkpoint_advances_only_after_page_yielded had a
handler that unconditionally returned a next_path → infinite loop. The
assertion was always about "a cursor was saved after page 1", so I
changed the handler to return page 2 as empty-with-no-next, which
terminates the loop cleanly.

Test plan
---------
## Automated
- poetry run pytest -q  →  70 passed
- poetry run mypy broker_sync tests  →  Success: no issues found in 29 source files
- poetry run ruff check .  →  All checks passed!
- poetry run broker-sync --help  →  renders without crash; lists version + auth-spike

## Manual Verification
End-to-end against a live T212 key is in the next commit once the
CLI subcommand and pipeline land.

pyproject.toml

@@ -12,6 +12,7 @@ httpx = "^0.27"
 beautifulsoup4 = "^4.12"
 python-dateutil = "^2.9"
 typer = "^0.12"
+click = "<8.2"   # typer 0.12 uses make_metavar() without ctx; click 8.2 made ctx required
 
 [tool.poetry.group.dev.dependencies]
 pytest = "^8.3"